Abstract. The non-central chi-squared distribution can be used to calculate power for
tests detecting departure from a null hypothesis. Required sample size can
also be calculated because it is proportional to the non-centrality
parameter for the distribution. We demonstrate how these calculations can be
carried out in Stata using the example of calculating power and sample size
for case–control studies of gene–gene and gene–environment
interactions. Do-files are available for these calculations.